Marine seismographic prospecting



1970 FR ANK M DONALD MURRAY 3,493,074

' MARINE SEISMOGRAPHIC PRQSPECTING V Fi1ed March 5. 1968 United StatesPatent 3,493,074 MARINE SEISMOGRAPHIC PROSPECTING Frank McDonald Murray,Kilmarnock, Scotland, assignor to Imperial Chemical Industries Limited,London, England, a corporation of Great Britain Filed Mar. 5, 1968, Ser.No. 710,616 Claims priority, application Great Britain, Mar. 10, 1967,11,316/ 67 Int. Cl. G01v l/38 US. Cl. 181-5 11 Claims ABSTRACT OF THEDISCLOSURE An assembly for marine seismographic prospecting comprises aplurality of lengths of fuse cord and a plurality of lengths ofnon-explosive cord, each of the latter being connected between two ofthe lengths of fuse cord. In use drag elements are attached to thenon-explosive cords so that sequential lengths of fuse cord will bedragged into the water after the initial drag element is thrown into thewater from a moving boat.

This invention relates to a method of laying elongated explosive chargesunderwater in marine seismographic prospecting operations and to anassemblyof explosive charges for use therein.

It has recently been proposed to use elongated explosive charges formarine seismographic prospecting in order to diminish the effect on theseismographic record of the expanding and contracting gas bubbleproduced by the explosion. One form of elongated explosive charge whichis favoured for this purpose is detonating fuse-cord in lengths of atleast 15 metres comprising a core of a granular high explosive such aspentaerythritoltetranitrate (PETN) or cyclotrimethylenetrinitramine,surrounded by textile wrappings and encased in a flexible waterproofenvelope of synthetic plastic material. In the marine seismographicprospecting operations the elongated explosive is stored as a bulksupply on reels on a boat and each individual charge in withdrawn asrequired from the reel, cut from the supply, fitted with detonationinitiating means and fed into the water behind the moving boat. At leastthe initial portion of the elongated charge must be positively fed fromthe boat into the water until sufficient length is immersed for the dragof the water to pull the remainder from the boat.

It is an object of this invention to provide a method of layingelongated charges underwater for seismographic prospecting wherein allthe charge is pulled from the boat by the drag of the water and toprovide an assembly of charges whereby this may be achieved.

In accordance with this invention a method of laying elongated explosivecharges consecutively in position for firing in marine seismographicprospecting comprises immersing in the water at a prospect area a firstdrag elesufficient length to prevent communication of detonation fromone charge to the next.

Each drag element may be in the form of a non-explosive cord which maybe joined at its ends to consecutive explosive charges. In a preferredform having improved drag, a vane, which may be in the form of a disc orfunnel, is attached to the cord to present a resistance areaperpendicular to the cord.

The elongated explosive charges may conveniently be lengths ofdetonating fuse-cord, and in this case the charges are convenientlyjoined together by intermediate lengths of non-explosive cord or stringwhich are preferably joined to the fuse-cord by crimped metal sleeveJoints.

The arrangement of charges may advantageously be stored on reels whichmay be mounted for rotation on an axle to permit the charges to bewithdrawn as required.

The charges are conveniently fired by means of detonating initiatingmeans comprising an electric detonator and for this purpose a longfiring cable terminating in an uninsulated hook may be trailed behindthe boat so as to sink the charge to the required firing depth. A metalslipring on the cable is attached to one terminal wire of the electricdetonator and is also attached by a tie member to an explosive charge,the tie member being arranged to prevent strain on the terminal wire.The other terminal wire of the detonator is left free. When theexplosive charge is pulled from the boat the metal slip-ring slides downthe cable to contact and to engage and be held by the hook. The freeterminal wire trails in the sea to pro vide an earth return for thefiring circuit. The charge is thus held in position for firing and thefiring circuit through the detonator is ready for firing and may befired by applying an appropriate voltage to the cable.

When the first explosive charge is fired the drag element is lefttrailing in the water and the drag may be used to pull the next chargeinto the water.

The invention also includes a method of producing a seismic record ofunderwater rock formations which comprises laying an elongated explosivecharge at a pros ect area by the aforedescribed method, detonating theexplosive charge and receiving and recording the resulting seismic wavesreflected or refracted from the underwater rock layer interfaces.

Further features of the invention are included in the preferredembodiment which is hereinafter described, by

way of example only, with reference to the accompanying drawings whereinFIG. 2 shows diagrammatically. an explosive charge being towed behind aboat in position for firing.

ment attached to one end of an elongated explosive charge v.

on a moving boat, permitting said explosive charge to be drawn into thewater by the pull of the drag element, attaching to the other end ofsaid explosive charge detonation initiating means and a second dragelement, and permitting the second drag element to be drawn into thewater by the pull of the first drag element, said second drag elementbeing also joined to one end of the next consecutive explosive chargeand being of sufficient length to separate the ends of the consecutiveexplosive charges sufliciently to prevent communication of detonationfrom one charge to the next.

The assembly of elongated explosive charges for use in the inventioncomprises two or more elongated explosive charges joined one end of oneto one end of another by means of one or more intermediate drag elementsof The charge assembly shown in FIG. 1 comprises lengths of detonatingfuse-cord 11 and lengths of cotton string 12 arranged alternately withtheir ends joined by a crimped aluminium sleeve 13. The assembly isstored on a reel 14.

In using the assembled charge for seismographic prospecting the reel 14is mounted for free rotation around its axis on a boat 15 (shown in FIG.2) from which an insulated firing cable 16 terminating in a bare metalbook 17 is towed as the boat is moved over the prospect sea area. Afunnel-shaped metal drag member 18 is crimped on the end length ofstring 12 to augment the drag on the string 12, and this end length ofstring 12 is thrown into the sea. The first length of detonatingfuse-cord is thereby pulled into the sea, but before it leaves the boatit is temporarily stopped and an electric detonator 19 is crimped to itby means of a deformable metal clip 20. A metal slip-ring 21 which isaround the firing cable 16 is joined to the length of detonatingfuse-cord 11 by means of a cotton string 22 and a crimped metal sleeve13 and is also joined to a bared end portion of one of the terminalwires 2-4 of the detonator 19. A further drag member 18 is crimped onthe next length of string 12 and the assembly is released. The slip-ring21 slides along the cable 16 until it reaches the hook 17 Where it isarrested. On further movement of the boat 15 the string 22 becomes tautand the immersed charge is dragged at a fixed distance behind the boatas shown in FIG. 2 so that the terminal wire 24 is not tensioned Thesecond terminal wire 25 has a bared end portion which lies in the waterto provide an earth return for the firing circuit. The charge in thisposition is fired by applying a high voltage between the cable and theboat. On firing the length of detonating fuse-cord 11 is destroyed andthe next length of string 12 is free to be pulled away from the boatthereby to draw a further length of detonating fuse-cord 11 into theWater. Slip-rings 21 from fired charges are pushed off the end of thehook 17 into the water by subsequent rings 21 being moved into thefiring position.

The lengths of detonating fuse-cord may be varied over a wide range asdesired but lengths of 15 to 150 metres of a detonating fusecommercially available as Cordtex (registered trademark) containing -20g. of PETN per metre have been found convenient. The lengths of string12 may also vary widely but lengths of to 50 metres are usuallysufficient to separate the detonating fuse-cord charges sufiiciently toenable one charge to be fired without any portion of the next chargebeing in the water.

What I claim is:

1. A method of laying elongated explosive charges consecutively inposition for firing in marine seismographlc prospecting which comprisesimmersing in the water at a prospect area a first drag element attachedto one end of an elongated explosive charge on a moving boat, permittingsaid explosive charge to be drawn into the water by the pull of the dragelement, attaching detonation initiating means and a second drag elementto the other end of said explosive charge, and permitting the seconddrag element to be drawn into the water by the pull of the first dragelement, said second drag element being also joined to one end of a nextconsecutive explosive charge and being of sufficient length to separatethe ends of consecutive explosive charges sufiiciently to preventcommunication of detonation from one charge to the next.

2. A method as claimed in claim 1 wherein each drag element comprises anon-explosive cord.

3. A method as claimed in claim 2 wherein a vane is attached to the cordto present a resistance area perpendicular to the cord.

4. A method as claimed in claim 1 wherein the explosive charge comprisesdetonating fuse-cord.

5. An assembly of elongated explosive charges for use in marineseismographic prospecting comprising at least two elongated explosivecharges, each including a length of detonating fuse cord, joined one endof one to one end of another by means of intermediate drag elements ofsufficient length to prevent communication of detonation from one chargeto the next, each of said drag elements including a length ofnon-explosive cord joined to the fuse cord by crimped metal sleevejoints.

6. A method of producing a record of underwater rock formations whichcomprises laying an elongated explosive charge at a prospect area by amethod as claimed in claim 1, detonating said explosive charge andreceiving and recording the resulting seismic waves reflected orrefracted from the underwater rock layer interfaces.

7. An assembly of elongated explosive charges for use in marineseismographic prospecting comprising a pinrality of distinct elongatedexplosive charges and a plurality of elongated distinct intermediatedrag elements of sufiicient length to prevent communication ofdetonation from one explosive charge to the next, the opposite ends ofeach drag element being connected to one end of a different explosivecharge.

8. An assembly as in claim 7 wherein each drag element comprises anon-explosive cord.

9. An assembly as in claim 8 including a vane attached to thenon-explosive cord to present a resistance area perpendicular to thenon-explosive cord.

' 10. An assembly as in claim 9 wherein said vane is in the form of adisc or funnel.

11. An assembly as in claim 8 wherein each of the explosive chargescomprises a detonating fuse cord.

References Cited UNITED STATES PATENTS 2,609,885 9/1952 Silverman.

2,729,300 1/1956 Paslay et al.

2,790,388 4/1957 MacLeod 10221.8 X 3,064,568 11/1962 Ainslie et al.102-9 3,118,374 1/1964 Karpovich 102-216 X 3,125,859 3/1964 Dilbey102-13 X 3,354,826 11/1967 Axelson et al. 10210 X VERLIN R. PENDEGRASS,Primary Examiner US. Cl. X.R. 102 1, 13, 27

